Especially in the field of optical communications, wavelength conversion techniques that produce light with a wavelength differing from that of the incident light by means of higher-order interaction between substances and light have attracted attention. In such wavelength conversion techniques, methods for efficiently extracting light from the interior of the material following conversion include:    (1) a method which utilizes the birefringence of a crystalline material, and accomplishes phase matching of the input and output wavelengths by propagating light at a specified angle; and    (2) a method called “quasi-phase matching” in which periodic polarization inversion regions are formed on the light propagation path, and the difference in the phases of the input and output wavelengths is eliminated in approximate terms.
Of these two methods, the latter quasi-phase matching would appear to possess numerous advantages in adaptation for practical use, e.g., the permissible width of the operating wavelength and angle of incidence is large, the phenomenon known as “walk-off” in which the input and output lights travel along different directions does not occur, and the like; accordingly, this method has been the focus of various expectations.
The formation of a polarization inversion region in a wavelength conversion element utilizing a quasi-phase matching technique can be realized (for example) by using a ferroelectric material such as lithium niobate as the substrate material, patterning an electrode in the region where it is desired to accomplish polarization inversion using a photolithographic technique, and applying a high voltage to this electrode, so that partial inversion of the crystal axes is accomplished by means of the electric field.
Besides such a method in which a polarization inversion region is formed by applying a voltage to a ferroelectric material, a wavelength conversion element in which a polarization inversion region is formed by using quartz (which is not a ferroelectric material) as the substrate, and applying stress, has been proposed in recent years (S. Kurimura, R. Batchko, J. Mansell, R. Route, M. Fejer and R. Byer: 1998 Spring Meeting of the Japan Society of Applied Physics, Proceedings 28a-SG-18).
This wavelength conversion element using quartz as the substrate material shows a light resistance that is at least 100 times greater than that of an element using a ferroelectric material as the substrate. Furthermore, the lower-limit wavelength at which the element is transparent is around 150 nm, while the same wavelength is 350 nm in the case of lithium niobate. Consequently, the following advantage is obtained: namely, light at wavelengths that conventionally could not be used, and in particular, even light at a wavelength of approximately 193 nm, which is comparable to that of an ArF excimer laser, can also be used.
Incidentally, wavelength conversion techniques are based on the mutual interaction of higher-order light and substances, and in order to obtain a high conversion efficiency, it is desirable that the energy density of the light within the wavelength conversion element be high. In cases where lithium niobate, which is a ferroelectric material, is used as the wavelength conversion material, a method that is widely practiced as a method that uses light with a high energy density is a method called the “proton exchange method” in which the refractive index is raised by replacing some of the lithium in the substrate with protons in high-temperature molten benzoic acid as shown below. This is a method in which a portion with a high refractive index is formed in the substrate by the proton exchange method, a waveguide is formed in this portion, and light is confined into this waveguide.LiNbO3+(C6H5COOH)x →Li1−xHxNbO3+(C6H5COOLi)x 
In concrete terms, after a polarization inversion region is formed by the application of an electric field, an aluminum thin film is formed on the surface of the substrate, with the region in which it is desired to form a waveguide left “as is”. The formation of the aluminum thin film is accomplished by an ordinary lift-off process. After being masked with aluminum, the substrate is immersed in benzoic acid heated to a temperature of 350° C. to 400° C., and is allowed to stand for a specified time, so that the proton exchange process is promoted. Following this proton exchange, the aluminum is removed by etching. The region into which protons have been exchanged has a higher refractive index than the surrounding regions, so that an optical waveguide in which light is confined and propagated is formed. Thus, light can be confined and propagated inside a periodic polarization inversion region, so that a high conversion efficiency can be obtained.
However, in the case of a quasi-phase matching element using quartz, the following problem is encountered: namely, waveguides cannot be formed using such a proton exchange process, so that the confinement of the light is impossible; as a result, a high conversion efficiency cannot be obtained.
The present invention was devised in light of such circumstances; the object of the present invention is to provide a wavelength conversion element in which a waveguide that confines light can be formed, so that a high wavelength conversion efficiency can be obtained, even in a quasi-phase matching element using quartz.